Hi Scott,
The question of licensing depends on where you live.  Some states may prohibit
other-than-commercial dredging operations altogether.  Others may prohibit the
taking of commercially important species at all, or the taking of them without
a commercial license.  You'll have to check with your local fisheries
department.
Your second question brings up an important point which I neglected to mention
in my earlier post.  If you are using a winch, either a power winch or a hand
winch, to retrieve the dredge, the tension of the tow line during actual
dredging should not be on the winch itself.  You should have a strong metal
cleat, designed for tying off a rope,  attached firmly to the boat (preferably
with bolts, not screws that can pull out).  While dredging, the tow line
should be tied off on the cleat.  Once the dredge is full, or the dredge run
is completed, allow a bit of slack to develop in the line, BEING EXTREMELY
CAREFUL NOT TO LET THE TOW LINE BECOME ENTANGLED IN THE BOAT PROPELLOR, then
untie the line and use the winch.
It is also important, if you are using an outboard motor, not to have a bigger
motor than you need.  You can only dredge at very slow speed - otherwise the
dredge comes up off the bottom.  Therefore, a small motor is adequate for the
actual dredging.  Of course, a bigger motor gets you to the dredging site
faster.  However, you have noticed that hanging an outboard motor on the stern
of a boat pushes the stern down somewhat.  You have also noticed that running
the motor pushes the stern down even more.  As you might imagine, towing a
dredge along the bottom pulls the stern down somewhat too.  Get the picture?
Combine a big motor and a heavy dredge with a small boat, and you may
push/pull the stern of the boat right under the surface.  This is especially
true in the scenario you mentioned - if the dredge suddenly gets caught on a
large rock or other immovable object.  So make sure you have a good clearance
between the top of your stern and the surface of the water while dredging.
Some dredgers use a breakaway device on one side of the dredge.  For example,
with a rectangular dredge, you can attach the upper and lower corners of one
side to the tow line with strong rope or light chain, but attach the other two
corners with heavy twine.  The theory here is that if the dredge gets hung up,
the twine will break, allowing the dredge to slide around the obstacle.  Of
course, after that it has to be retrieved, and new twine installed.  I don't
do this myself.  On the few occasions I have gotten hung up on the bottom, I
have gotten loose by circling back and pulling in the opposite direction.
Again, make sure the rope stays away from the propellor, or an annoying
problem may turn into a really bad problem!
One final note - if the bottom is sloped (for example if you are dredging
toward shore or away from shore), it is much easier to keep the dredge on the
bottom if the dredge is moving uphill rather than downhill.
Regards,
Paul M.